The invention relates generally to a suspension system for protection of devices mounted in an external housing and more specifically to a suspension system for a ruggedized radiation detector.
Radiation detectors typically include a light detecting and quantifying device, such as a photo-multiplier tube, and a scintillation element that may be a crystal or suitable compounded element. The scintillation element functions by capturing radiation from its surroundings and converting that energy into light. The radiation may be ambient background radiation or radiation emitting material that has been transported into proximity with the radiation detector.
Light generated within the scintillation element, as a result of impingement by radiation, is transmitted through an optical window into the photo-multiplier tube. The light impulses are transformed into electrical impulses that are transmitted to an instrumentation system. Optical coupling elements are normally used between the scintillation element and the light-detecting element in order to achieve better light transmission, and may be used to provide dynamic isolation between the scintillation element and the light-detecting element.
Portal radiation monitoring has taken on increased significance with the contemporary potential for illegal transportation of nuclear weapons, dirty bombs and other illicit radioactive materials. Effective portal monitoring for radiation offers a way to identify and thwart the improper transport and use of these radiation emitting materials. However, as the means for transport of the radioactive materials is multifaceted, portal monitoring must be applied to a broad set of transportation schemes. Among the modes of transport for which portal radiation monitoring must be conducted are shipments by sea, train, vehicle and by personnel. Consequently, the portal radiation monitoring equipment, including the radiation detectors, are exposed to a broad variety of environmental conditions.
Existing portal monitoring radiation detectors are often subjected to varying degrees of shock or vibration during their normal usage. In some cases, the degree of shock or vibration exposure may be quite severe. It is advantageous to protect radiation detectors so that they will not suffer any deleterious effects from the shock and vibration. Examples of these effects may include high background counts, noise in the detector's response spectrum, and even breakage of the detector. Typical methods of protecting these detectors have comprised the use of thick elastomers, foams, etc.
Existing shock and vibration isolation systems typically consist of either an elastomeric boot that is placed around the radiation detector or a foam pad that is wrapped around the radiation detector. Due to size constraints in portal monitoring radiation detectors, even these methods may not commonly be employed. In many cases, the crystal is simply wrapped in a reflective material and then inserted into a 1 mm thick stainless steel housing. The crystal is typically in the shape of a 4 inch×4 inch rectangle that is 16 inches long. Additionally, the crystal may be in other shapes, commonly including a 2 inch×4 inch rectangle that is 16 inches long.
Typically, soft elastomeric materials are used to provide cushioning, the greater the anticipated shock, the thicker the elastomer to be used. This material can be shaped in the form of boots, or sheaths, and may be achieved by potting the vibration sensitive element in an elastomer. Elastomers tend to change shape after large temperature changes due to their high coefficient of thermal expansion or due to high mechanical loading.
In one such method of building radiation detectors, a sodium iodide crystal is suspended in a metal housing a Teflon® boot. Teflon® tape is wrapped around the outside of the scintillation crystal until the dimensions match the inside of the housing. The wrapped crystal is then inserted into the housing.
Another assembly method uses foam. The sodium iodide first needs to be wrapped with a reflector to improve the internal photon reflection. After wrapping, the crystal is then packaged into the housing various systems secured inside the water resistant housing.
A larger scintillation element increases the cross-section and therefore increases the probability that a gamma ray or neutron will pass into the element. Also, a greater thickness increases the probability that the incident radiation will produce a scintillation, rather than just passing through the element. Further, the materials surrounding the scintillation crystal may attenuate the incident radiation. The thickness and characteristic of the protective materials and housing can adversely influence the sensitivity of the device.
There are numerous patents issued for various types and designs of radiation sensors utilizing the aforementioned types of suspension systems. A different concept for protection of a detector crystal incorporates the use of metallic flat springs around the circumference of a cylindrical scintillation crystal. In Frederick et al. (U.S. Pat. No. 5,962,855), a radiation detector of roughly cylindrical shape with a sidewall axial restraint surrounding the detector and radial springs placed outside the restraint provide stiff restraint in the axial and radial directions. Also in Frederick (U.S. Pat. No. 6,355,932) a first set of elongated, radial springs are located about the circumference of a light detector, radially between the housing and the light detector and a second set of similar radial springs are located about the circumference of a radiation detector where the detectors are cylindrical-shaped. These Frederick patents are assigned to General Electric and commonly employed in the manufacturing and design of cylindrical scintillation detectors for use in oil and gas exploration. However, these patents provide springs circumferentially across the entire face of the detector, thereby partially shielding the detector from incident radiation that it seeks to measure.
Accordingly, there is a need to provide a ruggedized suspension system for protecting the scintillation crystal in a square, rectangular or other polygonal shape from mechanical shock, vibration and temperature induced forces. Further is the need to provide a suspension system that minimizes shielding of the detector from the incident radiation that it seeks to measure.